Yoghurt, pudding, butter, and other semiliquid materials, typically foodstuffs, are normally packaged in a cup having a circular upper edge that is sealed to the periphery of a circular foil disk. The filling and sealing are done under substantially sterile conditions so that the hermetically packed material will not spoil unless opened. Typically the cup is made of a thermoplastic synthetic resin and the foil cover is coated with a thermoplastic resin to allow ultrasound or the like to be used to weld the two together. Such a package can be made at very low cost yet can provide a sealed sterile containment for the foodstuff.
In a standard mass-production packaging operation, which for instance can be producing 25,000 to 33,000 such packages per hour, it is inevitable that some of the cover disks will not be sealed perfectly to the respective cups. The covers can themselves be torn or otherwise perforated, and something such as for instance the foodstuff being packaged can have gotten on the rim and prevented the cover from being sealed at this location. As such an imperfect seal can permit the product to spoil, it is essential to cull out any packages that do indeed have imperfect seals.
U.S. Pat. No. 4,730,482 of Cerny discloses a cover- checking device. In this arrangement a heated plunger is pressed pneumatically with a predetermined fixed force against the cover to be tested. The heat of the plunger is transmitted through the cover to the air trapped underneath it, and this air expands. If the package is perfectly sealed the result is a lifting of the cover or at least increased force pushing the plunger up. If the package has a leak the heated air escapes and there is no change in pressure on or position of the cover disk. In a motion-detecting arrangement a highly sensitive fiber-optical position detector associated with the plunger can detect the tiny, 0.5 mm to 1.0 mm, displacement caused by the heated air in the container and can therefore distinguish between sealed and leaky packages.
This system has several considerable disadvantages, the main one being that it is very slow. The time necessary to move the equipment into place and then heat the air in the container enough to get a measurable reaction is considerable. In a large mass-production packaging plant where the packages move eight abreast along the conveyor line this type of bottleneck is a considerable problem. Also, some foods like butter cannot be subjected to the heat, and some inks used on the cover foils are similarly sensitive to heat. Finally this type of device cannot be used at all with products like pudding that are already hot.
German patent document No. 2,837,997 of M. Gauchel describes a method of testing the airtightness of a container by measuring its size while internally pressurizing it. This principle is hard to apply to food containers as any simple procedure for internally pressurizing the sealed container would inherently compromise the seal.